Understanding Barrier Property Testing for Custom Mylar Bags
To test the barrier properties of a custom mylar bag, you need to perform a series of standardized laboratory tests that measure its resistance to gases like oxygen and water vapor. The most critical and widely accepted methods are oxygen transmission rate (OTR) testing and water vapor transmission rate (WVTR) testing, which provide quantitative data on how well the bag’s layered structure, particularly the metallized polyester (mylar), protects its contents. This isn’t a simple at-home experiment; it requires specialized equipment to generate accurate, actionable data that ensures your product’s shelf life and integrity. The process involves understanding the bag’s construction, selecting the right tests based on your product’s needs, and interpreting the results against industry benchmarks.
Why Barrier Properties Are Non-Negotiable
Think of barrier properties as the bag’s immune system. For products sensitive to oxygen (like coffee, nuts, or pharmaceuticals) or moisture (like electronics or dehydrated foods), a failure in the barrier means a failure of the product itself. Oxygen can cause oxidation, leading to rancidity and spoilage, while moisture can result in mold, clumping, or corrosion. Mylar bags are excellent barriers because they are typically made from a flexible laminate. The core layer is often a biaxially-oriented polyester (boPET) film that’s vacuum-metallized with a thin layer of aluminum. This metal layer is the primary barrier. However, this structure is then laminated to other layers, like polyethylene (PE), which provides the heat-sealing capability. Any weak point—especially the seals—can compromise the entire system. Testing validates that the lamination process, the metallization quality, and the seal integrity are all up to par.
Core Laboratory Testing Methods
This is where we move from concept to hard data. Reputable packaging labs use instruments that adhere to international standards set by organizations like ASTM International. Here’s a breakdown of the two primary tests.
Oxygen Transmission Rate (OTR)
OTR measures the volume of oxygen that passes through a specific area of packaging material over a 24-hour period under controlled conditions of temperature and humidity. The unit of measurement is cubic centimeters per square meter per 24 hours (cc/m²/24hrs). A lower number indicates a better barrier. Standard test conditions are 23°C (73°4F) and 0% relative humidity. For a high-quality metallized mylar bag, you should expect an OTR value of less than 1.0 cc/m²/24hrs. Some premium bags achieve rates as low as 0.1 or even 0.05 cc/m²/24hrs, which is considered a high-barrier or ultra-high-barrier performance. The test involves placing the film sample between two chambers. One chamber has flowing oxygen, and the other has a carrier gas like nitrogen. As oxygen molecules permeate through the film, they are carried by the nitrogen to a sensor that quantifies the amount.
Water Vapor Transmission Rate (WVTR)
WVTR, also called moisture vapor transmission rate (MVTR), measures the amount of water vapor that passes through the material under specific conditions, typically 38°C (100°F) and 90% relative humidity. The standard unit is grams per square meter per 24 hours (g/m²/24hrs). Again, a lower number is better. A good metallized mylar bag will have a WVTR of less than 0.1 g/m²/24hrs, and often as low as 0.02 g/m²/24hrs. The test method is similar to OTR but uses a water vapor gradient instead of an oxygen one. A dish containing a desiccant (drying agent) is sealed with the film sample and placed in a controlled-humidity chamber. The weight gain of the dish over time indicates how much moisture has passed through the film.
| Test Type | What It Measures | Standard Units | Typical Conditions | Excellent Mylar Bag Performance |
|---|---|---|---|---|
| Oxygen Transmission Rate (OTR) | Flow of oxygen gas | cc/m²/24hrs | 23°C, 0% RH | < 1.0 cc/m²/24hrs |
| Water Vapor Transmission Rate (WVTR) | Flow of water vapor | g/m²/24hrs | 38°C, 90% RH | < 0.1 g/m²/24hrs |
Seal Integrity: The Critical Weak Point
You can have the best barrier film in the world, but if the seals are weak, it’s all for nothing. The heat seals along the edges of the bag are potential failure points. Therefore, testing the integrity of these seals is as important as testing the film itself. Two common methods are:
Burst or Creep Testing: This test measures the strength of the seal by applying internal pressure to the sealed pouch until it fails. The failure point is recorded in pounds per square inch (PSI) or kilopascals (kPa). This tells you how much pressure the seal can withstand, which is crucial for products that might be transported by air (where cabin pressure changes) or for vacuum-sealed packages.
Dye Penetration Testing: This is a destructive test that checks for pinhole leaks in the seal. A seal is submerged in a dye solution under vacuum. If there are any channels or gaps in the seal, the dye is drawn in. The seal is then cut open and inspected for dye penetration. It’s a pass/fail test that’s excellent for quality control checks on production runs.
Accelerated Shelf-Life Studies
While OTR and WVTR give you fundamental data, an accelerated shelf-life study simulates real-world aging in a compressed timeframe. You fill bags with your actual product and place them in environmental chambers that subject them to elevated temperatures and humidity levels. By measuring product degradation (e.g., checking for rancidity in oils or moisture content in dry goods) over weeks or months, you can extrapolate how the product will perform under normal storage conditions for a much longer period, often a year or more. This is the most comprehensive test because it evaluates the entire package-product system.
Practical Considerations and Working with Labs
Unless you have a six-figure budget for analytical equipment, you’ll be working with a third-party testing laboratory. When you approach a lab, be prepared to provide them with flat, unused samples of your mylar bag material, not the finished, filled pouches. They need a flat section to mount in their testing equipment. Clearly communicate what you are packaging, as this will determine the most important tests and the acceptable performance thresholds. For instance, packaging for sensitive electronics might prioritize WVTR, while gourmet coffee would prioritize OTR. The cost for these tests isn’t trivial; a single OTR or WVTR test can range from $200 to $500, and a full accelerated aging study can run into thousands of dollars. However, this investment is minor compared to the cost of a product recall or brand damage due to packaging failure.
It’s also wise to establish a baseline. If you’re switching suppliers or materials, test the new bag against your current one. This comparative data is incredibly valuable. Furthermore, don’t assume all mylar is created equal. The thickness of the metallization, the quality of the lamination, and the consistency of the seal can vary significantly between manufacturers. Rigorous testing is the only way to verify you are getting the barrier performance you are paying for. Always request certified test reports from the lab that specify the standard test methods used (e.g., ASTM D3985 for OTR or ASTM F1249 for WVTR). This documentation is essential for your quality records and for providing proof of performance to your customers or regulatory bodies.